Has anyone got the idea how to calculate the bolt torque value for U tube heat exchanger flanges?
Formula for Torque as below:
Torque = kdW/n
k = friction factor,
d = bolt root diameter,
W = Tightening force,
n = no of bolt.

I don't know how to calculate the tightening force W. Is it take the
max(Wm1,Wm2) value which is read from PV Elite flange calculation?

Hello Lim

There are published values in a number of publications. The 'Pressure Vessel Design Manual' 3rd edition by Dennis R Moss is one of them. There does not seem to be a definitive method for relating torque to bolt stress. There are other issues that affect the relationship between torque and stress. For example: Are the bolt Teflon coated? Is a lubricant used? Are washers employed? The may be other things that affect the stress of the bolt. This answer may not be satisfactory.

A method that has been used is to have a partially hollow bolt with a strain gauge installed in it. As the bolt is torqued up, the torque and the bolt stress can be correlated. That may be extreme, but I have seen this done at a flange seminar.